1. Field of the Invention
The present invention relates to a lubricating oil composition for automatic transmissions. More particularly, the present invention is concerned with a lubricating oil composition for automatic transmissions which is excellent in the performance of preventing shudder of friction materials in a clutch portion for automatic transmissions, enhanced in the capacity of transmission torque, and at the same time capable of maintaining the performance of preventing said shudder and the enhanced capacity of transmission torque for a long period of time.
2. Description of the Related Arts
There has been introduced in recent years, a slip control system of operating a lockup mechanism even in the region of low velocity of automobile transmissions in order to decrease a power transmission loss in automatic transmissions and improve fuel consumption. In the case, however, where a lockup mechanism is operated in the region of low velocity of automobile transmissions, there is frequently caused the problem that abnormal vibration of an automobile body called shudder takes place in lockup friction surfaces. In such circumstances, there is eagerly desired the development of a lubricating oil composition for automatic transmissions which is capable of preventing said shudder and maintaining the transmission torque at a high level for a long period of time.
It is known that in order to prevent the above-mentioned shudder, there is required such a .mu. (friction coefficient) --V (sliding velocity) characteristics that the friction coefficient increases with an increase in the sliding velocity. There has heretofore been suggested the use of a friction modifying agent such as a phosphoric acid ester and an amidocarboxylic acid. For instance, Japanese Patent Application Laid-Open Number 305082/1995 (Heisei-7) discloses a lubricating oil composition for automatic transmissions which comprises a base oil blended with 1 a zinc dithiophosphate and/or a basic zinc dithiophosphate, 2 at least one compound selected from the group consisting of phosphoric acid esters, acidic phosphoric acid esters and phosphorous acid esters and 3 an organic friction modifying agent in which a specific bond and/or a functional group are introduced in its molecules. The aforesaid composition, although being favorable in the initial performance of preventing shudder, suffers from the disadvantage that in the case of long-term usage, the zinc dithiophosphate is stuck to the surface of a wet clutch and markedly shorten the duration of preventing the shudder, thus making it impossible to maintain the initial performance. In addition, Japanese Patent Application Laid-Open Number 254196/1988 (Showa-63) discloses the use of a friction modifying agent such as a phosphoric acid ester, a fatty acid ester or a fatty acid amide as an additive for a lubricating oil composition for automatic transmissions. The above-mentioned composition, however, is insufficient in the initial performance of preventing the shudder.
That is to say, the use of the above-disclosed compound brings about such problems as insufficiency in the performance of preventing shudder, lowering of the friction coefficient (.mu.) in a wet clutch portion and/or the shortage of capacity of transmission torque. Further, the problems still remain unsolved in that persistency of favorable .mu. -V characteristics is insufficient as compared with the service lifetime of automobile bodies, and frequent renewal of lubricating oil is forced to carry out.